Conventionally, for brazing for heat exchangers made of stainless steel used for, for example, refrigerant evaporator/condenser, EGR cooler, or hot-water supply applications, copper brazing has been widely applied. However, in recent years, heat exchangers have been required to have higher efficiency, leading to higher-temperature environments, and thus brazing with a copper brazing material has been becoming insufficient in terms of durability.
Thus, replacement with a nickel brazing material, which has corrosion resistance, and oxidation resistance higher than those of a copper brazing material, has been considered. Examples of nickel brazing materials used for joining for a heat exchanger made of stainless steel include BNi2, BNi5, and BNi7 defined in JIS Z 3265:1998, “Nickel Brazing materials”.
However, there are problems as follows. BNi5 has a high melting temperature, and thus brazing is performed at a high temperature of 1200° C. or more, resulting in significant thermal effects on the stainless steel base material. BNi2 contains B, and B enters the grain boundary of the brazed stainless steel base material, resulting in a decrease in the strength of the base material. In addition, BNi2 has a low Cr content and thus is inferior in terms of corrosion resistance and heat resistance. BNi7 has a low melting temperature, but its material strength is low. Therefore, joining strength after brazing is low.
Thus, in order to solve such problems, in recent years, for example, novel brazing materials have been proposed such as those described in Patent Documents 1 to 6 below.
The brazing materials described in Patent Documents 1 to 4 below all contain Ni as a main component together with Cr, Si, P, and the like, and they have sufficient joining strength. However, they are either high-melting-point brazing materials having a melting temperature of more than 1000° C. or brazing materials having insufficient corrosion resistance. In addition, although brazing materials having a melting temperature of 1000° C. or less are described in the examples of Patent Documents 5 and 6 below, they have problems of having insufficient material strength or corrosion resistance or containing B that affects the strength of a base material.
Thus, the general-purpose brazing materials defined in JIS Z 3265:1996 “Nickel Brazing materials” and the nickel brazing materials described in Patent Documents 1 to 6, which have been used to date depending on the usage environments, have the problems mentioned above. Under present circumstances, there is no nickel brazing material proposed, which has heat resistance/corrosion resistance and moderate material strength, and also has all the characteristics that allow for brazing at relatively low temperatures.